This invention relates to defragmentation (defrag) in a storage system. As background art of this technical field, there is known U.S. Pat. No. 8,359,430 B1. A storage apparatus disclosed in U.S. Pat. No. 8,359,430 B1 carries out normal reference processing in response to a reference request from a host, and obtains continuous data (logically continuous data) in a content. At this time, when physical locations of the data are not continuous, the storage apparatus writes the data into physically continuous areas during next writing into a disk. Further, in U.S. Pat. No. 8,359,430 B1, there is disclosed a method involving first reading, at the time of a host update request, not only write data thereof but also logically continuous data on the periphery of the offset of the write data, and then wiring the read data in a disk.